How can activity numbers go down

Same date range but not the same time range. 

Ok, I get what you’re saying. That if there was an activity on 17th December that dropped off the end of the rolling 30 days the Low Aerobic value could drop. 
BUT I didn’t do any activities on 15th, 16th, 17th or 18th December so I still don’t see how the number could go down? 

Do you only do cycling activities? 

Yes, only cycling and they are all uploaded to Garmin Connect and as you can see from the screenshot above, I did no activities 15-18th December so hence I don’t know how the number can go down if the range we’re talking about is 17th Dec -13th Jan. 

Then it could be that it needs an activity to update the values.